Question: How do I use a custom validator in a gridview?

I have a gridview where I am trying to validate two concatenated fields with two other concatenated fields. The fields I concatenate build a date/time string. Each set of fields has a dropdown, with a date, in mm/dd/yyyy format, and a textbox, which uses a RegularExpressionValidator to ensure the time is entered in 12-hour time, in the HH:MM am/pm format. The fieldnames I use are DDLDateIn and txtTimeIn, and DDLDateOut and txtTimeOut. If anyone has any suggestions on how to use a custom validator control inside of the GridView, I'd love to hear them. Thanks.